I have 2 thermistors, these cable into a controller unit with some sort of logic programmed into it to switch on and off a pump.

The problem for me is I want that data wirelessly, but I can't figure where I'd get the data out from the circuit design they have.

I'm not sure if I could just piggy back on the same cabling, I think this would cause feedback back into the controller unit (or vice versa), so I would prefer to have the ability to switch between them.

Any ideas on how I can have a digital switch between the thermistor and the arduino?

Start with this - find a spot where you can put a voltmeter on one or both thermistor pins and see what kind of voltages are coming out. I'd be surprised if its anything but a DC valueYou may be able to just read that voltage into an arduino analog input pin, then send the value out via RF Transmitter module with virtual wire to wherever your receiver is:

Yes, you'll have to do some controlled temperature experiments.There is a standard equation for how thermistors change resistance with temperature, but you do not have access to use that resistance, only the outcome of its change - and in this case even that cannot be assured as you don't know if the design keeps the source voltage constant across the part.So, take some measurements, make yourself a graph.

The easy solution for me is to just record the voltage outcomes, build a table and make conclusions of temperature based on the voltage outcome.

However, is there anyway of making the connection to the thermistor a switched one?

Is there anyway, I can have the thermistor cable go to two locations, even if only one could access it at any one time?(and if this is the case, couldn't that be electrically switched some how)?

1 thermistor - 2 devices using it..

What would happen if I joined two wires at the device end of the thermistor and tried to get resistance measurements (i.e. the thermistor is still attached to the controller, that's powered on, and I've got some cable from the arduino to the controller's thermistor joint)?

So you would have it electrically disconnected from the other circuit then?I would suggest a double pole, double throw relay (for each thermister).Have the thermister connected to the common leads, the NC leads would go to the existing circuit and the NO leads to your circuit.You would then need your own circuit for powering the relay and for powering the thermister.I have to go shovel some snow (northeast blizzard of 2010), can draw a simple schematic later if you need one.

I tried to use the arduino to read the voltages, but I turned the arduino off to see what happens if the battery is flat, this caused the readings to jump to wrong values and therefore would not serve the purpose I need.

I should add - the purpose of this is to monitor the temperatures of a solar hot water collector and water tank, I mainly need to know if it's been above 60oC at least once every 3 days, but also, would like to have the data logged too.

I've thought of using Dallas sensors on the pipes, but this was inaccurate, measuring the voltage outcome from the sensor circuit isn't going to work.

Adding another thermistor isn't going to work as there's no room to.

So this leaves really, checking to see if the relay will work without causing the pump to kick in and out (I think that will be an issue).

Hack the PIC microcontroller on the controller unit to give the data to the arduino, then go wireless from there. I don't know how PIC is coded or pin outs...

Make my own unit to do exactly what it does (monitors the thermistors, if there's a 12 degree difference, kick the pump in, whilst the tank is under a set value). The problem with this is I'd have to dig up a lot of information to get it done right.

The unit by the way is the "SolaStat-ST" controller, it's not the "Plus" version that has RS485 (which would make this so much easier).

I went thru all they had, I think you are best off using the relays to take your own readings unless you can get inside the controller and add an op-amp buffer there to feed into your own ADC.

Be interesting to see what your controller does when it gets an open circuit where the thermister is supposed to be. You can fake it out with - swap in a resister that is similar in value to when the system would think no action is needed for the time when you are getting your logging readings.